Computed tomography (CT) is a diagnostic imaging technique which measures, in its imaging process, the radiodensity of matter. Radiodensity of matter is typically expressed in Hounsefield Units (HU). Hounsefield Units are a measure of the relative absorption of computed tomography X-rays by matter and is directly proportional to electron density. Water has arbitrarily been assigned a value of 0 HU, air a value of -1000 HU, and dense cortical bone a value of 1000 HU.
Various tissues in the body possess similar densities. Difficulty has been encountered in generating by CT visual images of tissues which possess similar densities and which are proximate each other. For example, it is difficult to generate separate CT images of the gastrointestinal (GI) tract and adjacent structures, including, for example, the blood vessels and the lymph nodes. Accordingly, contrast agents have been developed in an attempt to change the relative density of different tissues, and thereby improve the diagnostic efficacy of CT.
A commonly used contrast agent for computed tomography, particularly in connection with scans of the GI tract for increasing the radiodensity of the bowel lumen, is barium sulfate. Barium sulfate increases electron density in certain regions of the body, and is classified as a "positive contrast agents."
Currently available CT contrast agents, including barium compounds, such as barium sulfate, suffer from various drawbacks. For example, the viability of CT agents is generally extremely sensitive to concentration. If the concentration is too low, little contrast is observed. If the concentration is too high, beam hardening artifacts result and are observed as streaks in the CT images. In addition, difficulty is generally encountered in visualizing the bowel mucosa with the currently available contrast agents.
Lipid compositions, for example, lipid emulsions and/or suspensions, have been formulated as contrast agents, particularly for the GI tract. Lipids inherently possess an electron density that is lower than water. Accordingly, lipid compositions are capable of decreasing electron density and are generally termed "negative contrast agents".
Lipid compositions are capable of providing enhanced visualization in CT scans. However, lipid-based contrast agents also suffer from various drawbacks. For example, compositions which comprise lipid alone are generally unpalatable which limits their use for oral applications. In addition, lipid compositions are typically expensive to formulate. Undesirable side effects can also be caused from the high concentrations of lipid which are frequently used in the lipid-based contrast agents to achieve adequate negative contrast in certain regions of the body, for example, the bowel lumen. Patients with pancreatitis, peptic or gastric ulcers, irritable bowel disease, Crohn's disease, or colitis are especially prone to such side effects. Furthermore, lipid-based contrast agents are typically perishable and thus possess a limited shelf-life.
Accordingly, new and/or better contrast agents for CT are needed. The present invention is directed to this, as well as other, important ends.